The term "two-cycle" engine is commonly regarded as applying to small engines utilized for powering small electric generators, lawn mowers, tractors, pumps, chain saws, motorcycles, as marine outboard motors, and many similar applications where small, portable power units are required. The chief identifying characteristic of these small two-cycle engines relates to the fact that the lubricating oil is mixed with the gasoline and engine lubrication is provided by this oil-fuel mixture. Consequently, the requirements of satisfactory oil for small two-cycle engines are quite different from the requirements of quality oil for automobile, truck and bus engines. An oil rated as excellent for automobile engines would most likely be rated as unacceptable for outboard-motor use.
Basically, two-cycle engine oils are mixed with gasoline, with the result that the oil supplies lubrication in a highly diluted form, and also passes through the engine very quickly to be burned in the combustion chambers. Thus, a quality two-cycle engine oil must be designed to supply proper engine lubrication in highly diluted form, and should burn cleanly with a minimum of soot, ash or carbon formation.
Recent public concern regarding engine oils centers around the harmful effects of oil to the environment. Especially in regard to two-cycle outboard marine engines, the need for a biologically degradable engine oil cannot be overstated. Lakes, rivers, and oceans are becoming increasingly polluted as water traffic increases. In some areas, bans have been placed on the use of large outboard engines. In response, consumers are becoming more environmentally aware, and much interest has been shown in the development of environmentally friendly products. As of yet, little development has occurred towards production of environmentally safe two-cycle engine oils in the U.S. Thus, a substantial need exists for a high quality biodegradable two-cycle engine oil which will be readily accepted by the consumer.
Versino et al., J Synth. Lubr. 1987, 4, 3, report biodegradability test results for a variety of esters using Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) values along with extractions of the samples which remained after testing. Versino et al., however, do not disclose or suggest any engine oil formulation or an engine oil formulation comprising the combination of a heavy ester oil in combination with a light ester oil.
In Kenbeek et al., J. Synth. Lubr. 1988, 5, 215, Kenbeek and van der Waal of Unichema International stated that esters can be formulated into biodegradable outboard engine oils which pass TC-W requirements. Due to the polar functionalities of esters, these compounds adhere to metal surfaces and have better lubricity than hydrocarbons. However, the Kenbeek et al. engine oil composition requires the presence of a solvent, creating the problem of lowered biodegradability and flash point. Further, Kenbeek et al. do not recognize that an unexpectedly superior engine oil lubricant may be obtained by blending a heavy ester oil in combination with a light ester oil.
A second paper by Kenbeek and van der Waal in 1991 reports the performance of ester-based two-cycle oils in air cooled applications as well as in TC-W engine tests. See Kenbeek et al., J. Synth. Lubr. 1991, 8, 83. However, as above, Kenbeek et al. do not disclose an engine formulation comprising the combination of a heavy and light ester oil.